Context is always so important in answering questions that try to analyse specific quotes from a scene. Act I scene 3 is important because in it Rosalind admits to Celia that she has fallen hopelessly in love with Orlando, and feels quite overpowered by the emotion. Celia's role in this scene is to try and cheer up her cousin by showing her how she can treat this "trouble" as a harmless diversion. Thus it is that, in response to Rosalind's expression of woe that her life is "full of briers," or thorns, Celia responds, urging her cousin to not view her worries as "briers" but as "burrs" that are small sticky seeds to be played with and that become stuck to our clothing. Rosalind's response rebuffs Celia's attempts to cheer her up, saying that the kind of burrs Celia talks about can easily be shaken off her clothing, but the burrs she is refering to are lodged deeply in her heart, and thus are not able to be disposed of so easily. Note how this imagery presents Rosalind as being helplessly in love. She is not able to do anything to help herself and get rid of her emotions or shake them off. They have well and truly stuck to her heart, the seat of her emotions, and she is left wondering what to do with them now.